This invention relates generally to the field of devices for cardiac surgery, and more specifically to devices for R-F ablation of cardiac tissue.
The present invention is directed toward treatment of tachyarrhythmias, which are heart rhythms in which an chamber or chambers of the heart exhibit an excessively fast rhythm. In particular, the present invention is directed toward treatment of atrial arrhythmias which result from the presence of macro and/or micro-reentrant wavelets (e.g. atrial flutter and atrial fibrillation) and treatment of ventricular tachycardia.
Therapies have been developed for treating tachycardias by destroying cardiac tissue containing identified ectopic foci or aberrant conduction pathways. A variety of approaches have been taken, including application of electrical energy or other forms of energy to destroy the undesired cardiac tissue. As examples, ablation of cardiac tissue has been accomplished by means of radio frequency electrical current, direct current, microwave energy, heat, electrical pulses, cryothermy, and lasers. At present, ablation using R-F energy is perhaps the most widely practiced in the context of ablation procedures that can be carried out by means of a catheter, inserted into the closed heart.
Most R-F ablation catheters employ electrodes which are intended to contact the endocardium of the heart, or, as in U.S. Pat. No. 5,083,565, are intended to penetrate the endocardium and enter the myocardium. In general, R-F ablation catheters are effective to induce small lesions in heart tissue including the endocardium and inner layers of myocardium, in the immediate vicinity of the electrode. However, the medical community has expressed a desire for devices which produce larger and/or longer lesions, to reduce the number of applications of energy (burns) required to effectively ablate cardiac tissue associated with more complex arrhythmias such as atrial flutter or atrial fibrillation and ventricular tachycardia.
R-F ablation causes tissue in contact with the electrode to heat through resistance of the tissue to the induced electrical current therethrough. The actual extent of heating is somewhat unpredictable. However, temperature tends to rise as the duration and amplitude of the R-F signal increase. Heating of the tissue beyond a certain point can cause dissection or charring of the tissue, resulting in a high impedance between the R-F electrode and the return electrode, which in turn leads to cessation of the heating process, and, in some cases, causes the electrode to stick to the charred tissue. One response to this phenomenon has been the inclusion of thermocouple within the ablation electrode, in conjunction with feedback control to modulate the R-F signal to maintain the electrode temperature at a set parameter. One such system is disclosed in U.S. Pat. No. 5,122,137.
Particularly in the context of treating macro and/or micro-reentrant atrial arrhythmias, it has been proposed to create elongated lesions, to define a line of tissue which blocks the passage of depolarization wavefronts. This has, in some cases been accomplished by means of a series of small, individual lesions, each produced by a separate application of R-F energy. As disclosed in U.S. patent application Ser. No. 08/302,304 by Mulier et al, for a "Method and Apparatus for R-F Ablation", it has been proposed that an elongated, coil electrode might instead be employed to produce an elongated lesion with a single application of R-F energy. An elongated coil ablation electrode is also disclosed in published PCT application No. WO94/11059 for a "Fluid Cooled Ablation Catheter, by Nardella.